1. Field of the Invention
The invention is directed to an apparatus for weighing moving postal items, particularly letters arranged standing on an edge.
2. Description of the Prior Art
Mail processing systems are usually composed of a mail separating device, a scale, a postage meter machine and a deposit means, as described in German OS 27 17 721 and German PS 37 31 525. The scale can be integrated in the postage meter machine, see DE-M 96 09 167.3 in the Utility Model Gazette of the German Patent Office dated May 24, 1997, Part 1a, goods class 18/02.
The weight range of the loads to be weighed usually amounts to 3 through 1000 g given a load thickness between 0.5 and 20 mm.
The postal items, referred to below as "letters", are deposited in a stack in the separating device and are separated and sealed as needed.
The letters are printed to identify a recipient and/or a postage amount according to the identified weight and the predetermined address in the franking and/or addressing machine.
In a known franking and/or addressing machine, as described in German PS196 05 014 and German PS 196 05 015, letters standing on an edge are continuously conducted past a print head. The printer head is stationarily positioned during the printing process. Weighing of the item is unaffected by this arrangement.
A scale is known from German Utility Model 296 01 418 that is particularly provided for weighing pieces of mail. The letters are weighed standing on an edge with this scale. This scale has a pan with a depression and an adjacent upright supporting surface. The letter has an edge seated in the depression and has a lateral surface lying against the supporting surface. The scale has a load cell which is resilient or deformable such that the load cell with a letter on the pan moves vertically downwardly to an extent that increases with the weight of the load on the load cell. The scale is further provided with transducer that generates an electrical signal corresponding to the vertical downward motion, this electrical signal corresponding to the load weight.
The pan is intentionally designed to have such a short horizontal extent that the letter must lie approximately centrally thereon in order not to fall off. This causes the center of gravity of the load to be located approximately at the horizontal center of the load cell and consequently the load exerts no significant tilting force on the load cell. The height of the load at this center likewise has no influence on the measured result.
For weighing items at high conveying speed, however, a pan is required whose horizontal length is at least equal to the maximum length that the item to be weighed exhibits in the conveying direction. This means that the known arrangement described above is only suitable for weighing loads at rest and the success of the weighing is dependent on the care of the operator in properly placing the item on the load cell pan.
In the franking system according to German PS 37 31 525, the letters are conveyed with horizontally disposed conveyor belts. The scale is arranged between the letter separating means and the franking machine and is linked via a transport control to the neighboring devices so that the letter transport is interrupted for the weighing process. The weighing time is dimensioned such that a stable weighing condition occurs during the interruption. An empirically determined value thereby forms the basis that a stable weighing condition is achieved when three successive count signals of the load cell are acquired within a predetermined time.
The load cell is secured on a base in order to reduce torsional stresses so that the horizontal middle line of the load cell is substantially aligned with the surfaces of the conveyor belts. Three conveyor belts arranged in parallel form a conveyor means that is seated in an elongated, small u-shaped profile member having a side wall flanged to the load cell. Detents are arranged under the profile member as overload protection.
Due to the implementation with three conveyor belts and the appertaining drive and deflection rollers, this arrangement exhibits a relatively high mass. Moreover, the profile member must have a wall thickness which is sufficient to assure a minimum stiffness, thus adding further to the mass. A high overall mass--a multiple of the maximum letter weight as experience has shown--has a correspondingly low natural frequency. As is known, the square of the natural frequency is inversely proportional to the mass and proportional to the stiffness of a body. If the weight measurement precision is to remain unchanged, the required weighing time, and accordingly the transport time across the scale, become longer as the natural frequency becomes lower. This type of coupling the profile member to the transport means causes a bending moment that correspondingly limits the selection of the load cell.
In the aforementioned German OS 27 17 721 letters are conducted across a horizontally disposed pair of an electronic scale with a transport means composed of a number of parallel belts, analogously to German PS 37 31 525. Weighing is carried out during transport, i.e. this is a type of scale referred to as a "dynamic scale". The transport speed and the length of the transport path are thereby dimensioned so that the necessary equipoise time of the scale is achieved, following which letter leaves the weighing region. Here, too, the transport means exhibits a low natural frequency. This requires a correspondingly long weighing pan and a low transport speed and/or a reduction of the upper weighing limit.
Both of the above-described scales are specifically designed for transporting the letters lying flat. Employment of these known transport devices for vertical letter transport would result in a further disadvantage of a higher measuring imprecision since the load center of the letters would then be significantly farther from the load cell, and the load arm larger.